The present invention relates to a process for the preparation of aminoaryl-sulphonic acids by reaction of arylamines and sulphuric acid at elevated temperature and under pressure.
Aromatic aminosulphonic acids can be obtained from arylamines and sulphuric acid at elevated temperature by the so-called "baking process" (Helv. Chim. Acta 15 (1932), 1372). In this, the corresponding aminoaryl-sulphonic acid is formed at elevated temperature from initially formed arylammonium hydrogen sulphates, either in substance or in an inert solvent with exclusion of water; the water is, as far as possible, rapidly and completely removed from the reaction mixture (Ind. Eng. Chem 42 (1950), 1746). This known "baking process" frequently yields dark-coloured materials with long reaction times and somewhat moderate yields, which contain aminoaryl-disulphonic acids and also isomeric aminoaryl-sulphonic acids as undesired by-products.
A process for the preparation of aminoaryl-sulphonic acids is described in EP No. 63,271, in which the conventional variation of the "baking process" is varied in that resulting water of reaction and water of dilution which is present in some cases is not, as far as possible, rapidly and completely removed from the reaction mixture but a part of this water is always left in the reaction mixture until shortly before the end of the conversion reaction and the reaction water is completely removed from the reaction batch only in the last reaction phase for completion of the reaction. This process is carried out under pressure. It shows considerable advantages compared to the conventional "baking process" described further above.